Fibrosis is an excessive growth of fibrous connective tissue in an organ, any part, or tissue thereof, for example in a liver, any part or tissue thereof, especially in response to an injury. Abnormal fibrosis occurs in chronic hepatic inflammations of various aetiologies such as in Hepatitis Virus and Schistosome infections. It was shown previously that certain subjects infected by Schistosomes are slow fibrosers whereas others are rapid fibrosers and that this depends in part on a major gene located on Chr 6q22-q23 (Dessein et al., 1999; Mohamed-Ali et al., 1999).
Schistosomiasis is caused by helminths that develop in the vascular system of their hosts and lay eggs that are for some of them carried over to the liver where they trigger inflammation in the periportal space. Since worms live for years in their human host, chronic liver inflammation associated with much tissue destruction is common in infected subjects. Tissue repair requires the deposit of extracellular matrix protein (ECMP) in the damaged tissues that are later on turned over and replaced by normal hepatocytes. In some patients ECMP accumulate in the periportal space forming fibrosis deposits that reduce blood flow causing varicose veins, ascites. After months or years of chronic or repeated injury, fibrosis becomes permanent and irreversible. Subjects die of the consequences of fibrosis.
In South countries, it is estimated that 5 to 10% of the 350 millions of infected subjects may develop severe hepatic fibrosis. There is no good marker allowing to predict and follow hepatic fibrosis progression in Schistosome infected subjects.
Diagnosis of hepatic fibrosis is mostly based on liver biopsy, elastometry (Boursier et al., 2008; Macias et al., 2008) and ultrasound analysis (Richter et al., 2001; Lambertucci et al., 2004; King et al., 2003).
Biopsies are obtained via percutanous, transjugular, radiographically-guided fine-needle or laparoscopic route, depending upon the clinical setting. Histopathological examination enables the clinician to grade the severity of necroinflammation and stage the extent of fibrosis. The Metavir scoring system attributes a score to the stages of fibrosis on a 1-4 scale as follows: F0=no fibrosis, F1=portal fibrosis without septa, F2=portal fibrosis and few septae, F3=numerous septae without cirrhosis, F4=cirrhosis (Bedossa et al., 1996). Liver biopsy is an invasive and costly procedure, and samples only a small portion of the liver. Thus it cannot afford a global assessment of hepatic fibrosis, and is subject to sampling variation and inter- and intra-observer error. In addition, liver biopsy is associated with significant morbidity of 3% and a mortality rate of 0.03% (Garcia-Tsao et al., 1993). Potential complications include local hematoma, infection and pain related to the biopsy.
Noninvasive tests (i.e., serologic markers, elastometry, ultrasound analysis) are also used but are not yet ready for routine clinical use.
Panels of blood markers have been tested mostly in patients with chronic hepatitis C or cirrhosis due to viral hepatitis C. These studies revealed that serum markers can rule on or rule out fibrosis in approximately 35% of patients (Sebastiani et al., 2006). However, when looking at patients individually, these markers could not reliably differentiate between the various stages of fibrosis. A more recent study incorporated three panels of serum markers to devise an algorithmic approach that improved diagnostic accuracy (Parkes et al., 2006). The three panels evaluated were the APRI (aspartate transaminase to platelet ratio index), the Forms' index (platelets, gammaglutamyltranspeptidase, cholesterol) and the Fibrotest (GGT, haptoglobin, bilirubin, apolipoprotein A, alpha-2-macroglobulin). An algorithm consisting of the APRI followed by the Fibrotest boosted the diagnostic accuracy of fibrosis to above 90%. This group estimated that use of this algorithm could obviate the need for up to 50% of liver biopsies. However, the individual stages of fibrosis are not distinguishable using this algorithm. The limitation of these serum markers is the possibility of false positives when there is highly active hepatic inflammation.
Fibroscan is another innovative approach to staging hepatic fibrosis, which is based on elastography, which provides rapid measurement of mean hepatic tissue stiffness (Ziol et al., 2005). A probe is employed to transmit a vibration of low frequency and amplitude into the liver. This vibration wave triggers an elastic shear wave, whose velocity through the liver is directly proportional to tissuestiffness measured in kilopascals (kPa). Sensitivity of the Fibroscan technique ranged from 79 to 95%, and specificity from 78 to 95%, compared to the liver biopsy. However, the limitations of this technique are associated with attenuation of elastic waves in fluid or adipose tissue, which would impair assessment of fibrosis in patients. In addition, Fibroscan is an extremely expensive instrument.
However, no efficient method exists to prognose the fibrosis progression and the treatment efficiency.
A large number of molecules have been tested for treatment of hepatic fibrosis. For example, corticosteroids have been used to suppress hepatic inflammation in autoimmune and alcoholic hepatitis (Czaja et al., 2003). Ursodeoxycholic acid has been proven to increase survival in PBC patients by binding bile acids, and thus also decreasing hepatic inflammation (Poupon et al., 1997). Neutralizing inflammatory cytokines with specific receptor antagonists (TNFalpha, IL-1 receptor antagonists) and prostaglandin E have been tested in murine models, but not yet in humans (Bruck et al., 1997).
Another attractive target in curtailing hepatic fibrosis is the downregulation of hepatic stellate cell activation. Interferon gamma is used in combination with ribavirin for therapy of hepatitis C infection. It is postulated that the antifibrotic effects of the interferons may be partially related to downregulation of stellate cell activation. This mechanism could explain the improvement in fibrosis described in patients with viral hepatitis C who do not have a virologic response to interferon alpha (Poynard et al., 1998).
Trials of antioxidants (nacetylcysteine, alpha-tocopherol) are currently underway in humans. Angiotensin II receptors are upregulated in stellate cell activation, thus angiotensin converting enzyme inhibitors and angiotensin receptor blockers have demonstrated antifibrotic activity in vitro and in animals. This has yet to be replicated in humans (Jonsson et al., 2001). Promoting matrix degradation through matrix metalloproteinases is an antifibrotic strategy shown to be beneficial in a murine model (Iimuro et al., 2003). Specific apoptosis of hepatic stellate cells is another interesting theoretical idea, but has not yet been investigated (Gressner et al., 1998). Treatments aimed at reversing the fibrosis are usually too toxic for long-term use (i.e., corticosteroids, penicillamine) or have no proven efficacy (i.e., colchicine).
In conclusion, efficient and well-tolerated antifibrotic drugs are currently lacking and current treatment of fibrosis is limited to withdrawal of the noxious agent.
It has been previously reported that fibrosis development is markedly influenced by a major locus on Chr 6q23 (Dessein et al., 1999). It has been also proposed that CTGF gene contributes to increasing fibrosis by synergizing with various pro-fibrogenic growth factors (Leask et al., 2006) including PGDF, VEGF and the master fibrogenic molecule TGF-β. More specifically CTGF acts as a TGF-β downstream modulator (Leask et al., 2006; Leask et al., 2003). It increases TGF-β binding to its receptor, interferes with the negative Smad-7 feedback loop on TGF-β. (Wahab et al., 2005); it inhibits receptor binding of the principal TGF-β antagonist BPM-7 (Abreu et al., 2002). An important consequence of CTGF action on TGF-β is the stimulation of the trans-differentiation of hepatic stellate cells and other parenchymal cells into ECMP producing myofibroblasts which is involved in progressive fibrotic process (Kalluri et al., 2003; Neilson et al., 2005). CTGF is also thought to increase ECMP networking trough its binding capacities to fibronectin domains on ECMP (Gressner et al., 2007; Yoshida et al., 2007). CTGF is produced by a variety of cells including hepatocytes (Kobayashi et al., 2005; Gressner et al., 2007), hepatic stellate cells, myofibroblasts and endothelial cells (Gressnet et al., 2008). An overexpression of CTGF transcripts have been reported in different tissues including liver (Rachfal et al., 2003) affected by fibrosis of different aetiological origin. Experimental work in rats has shown that inhibiting CTGF by siRNAs prevents or reduces tissue fibrosis (Li et al., 2006; George et al., 2007).
However, the genetic factors that control the human fibrosis susceptibility were not identified as well as their effect on fibrosis progression.